1. Field of the Invention
The invention relates to a catalytic process for complete removal of heteroatoms from heteroatom-containing aliphatic hydrocarbons using a nickel-containing zeolite catalyst and is particularly suited for hydrodehalogenation of polyhalogenated compounds. The process is characterized by high conversion efficiencies without rapid catalyst deactivation. Ratios of paraffinic and aromatic products may be altered by modification of reaction conditions.
2. Description of Related Art
Various heteroaliphatic compounds are widely used commercially and industrially as solvents or reactants in the production of plastics, herbicides and other products. Unfortunately, many of these compounds are toxic and have posed significant problems in disposing of waste and residual material.
Present methods of disposing of toxic materials, particularly low molecular weight chlorinated liquid compounds, are not satisfactory. Landfills are inadequately protected against runoff or leakage into underground water supplies. Incineration frequently does not provide complete breakdown and in fact may produce other toxic compounds which are discharged into the atmosphere. Moreover, complete combustion of some compounds often requires high temperatures to obtain desired efficiency, resulting in high expenditure of energy.
Among the most common waste products are haloorganic compounds, including chlorinated hydrocarbons. There have been efforts to Develop efficient processes for removing one or more chlorine atoms from these compounds, especially the volatile chlorinated organics. A proposed alternative to incineration is biological and chemical waste conversion. In one example, chemical destruction of tetra- and trichloroethylenes in solution was achieved by reductive dechlorination with sodium naphthalenide (oku and Kimura, 1990). Another example of chemical destruction is the high-temperature gas phase reductive dehydrochlorination of chloroform and 1,1,trichloroethane by direct reaction with molecular hydrogen (Chuang and Bozzelli, 1986).
The major difficulty encountered in chemical reductive dehydrochlorination of these solvents is that many industrially important polychlorinated compounds, such as carbon tetrachloride, trichloroethanes, trichloroethylene, etc., are difficult to completely dechlorinate using economically acceptable routes. In principle, a catalytic process offers possibilities for chemical conversions and a number of studies have focused on hydrodehalogenation. However, these studies have been limited to only partial removal of the halogen, as in dehydrochlorination of 1,1,2-trichloroethane to 1,1-dichloroethylene over alumina (Mochida et al., 1978) or to mixtures of 1,1- and 1,2-dichloroethylenes over an alkali ion/SiO.sub.2 catalyst (Gokhberg et al., 1989). Also, zeolite catalysts have been employed to achieve partial dechlorination of a variety of di- and tri-chlorinated hydrocarbons (Diesen, 1983). Attempts at more extensive catalytic dechlorinations with zeolite Y and mordenite catalysts have resulted in rapid catalyst inactivation due to deposition of carbonaceous material on the catalyst surface (Imamura et al., 1989; Weiss et al., 1982). As illustrated in the present application, shape selective zeolite catalysts, similar in composition to those employed by Diesen (1983) and Butter, et al. (1975) also exhibit rapid catalyst deactivation when employed under reaction conditions in which complete dechlorination of polychlorinated reactants is attempted.
In general, efforts to remove multiple chlorine atoms from hydrocarbon compounds have not been successful, particularly where zeolite catalysts have been used. To date, every reported catalyst study in which complete dehalogenation of polyhalogenated reactants was attempted has failed as a result of rapid catalyst deactivation. Major drawbacks of attempts to use catalytic methods include inefficiency of the dechlorination process, relatively rapid poisoning or inactivation of the catalyst and/or economically unacceptable routes of conversion.